Tips and tricks
Audio should not be a problem. Learn to adjust volumes in flash or simply edit the files outside of flash to normalize the volumes. Also, having the music turn down during dialogue helps a ton.
Visually, there's plenty that can be done, but i think the easiest way to make this look better is backgrounds. Take some time on creating an environment for your characters and the payoff is amazing! Now, be careful not to stray outside of the artistic style you're using here - i.e. don't stop using the hard edges and thin lines, but more things in the background and you'll find this looks a whole lot better.
Take some classes (online tuts even) on perspective and all that. I think your production is quite all right, you just need a little help on the look and sound of it!
Still needs work
I see you kept some of my suggestions in mind (I.E. the voice acting, background, and I noticed better perspective). I personally feel characters created with the line tool often look very flat and unprofessional. Try learning more technique with the brush tool. The story was somewhat more engaging, but I found many moments where I had no idea nor cared what was happening. For instance, the scene transitions that flash random characters made no sense. All in all, the whole movie still felt quite empty and lacking for large portions. Consider adding more atmosphere, such as sound effects not related to just the fight and perhaps more background activity to keep your viewer attentive.
Keep it up, I can definitely see improvements.